While some of Matt Thornton's early-season struggles have been due to bad luck, command issues have plagued him as well.
As soon as Thornton regains good command of his fastball, his luck will reverse and he'll be fine. What's happened with Thornton in his five appearances this year is that he's had the killer combination of bad luck and bad command—it's possible to have good luck with bad command, mind you, or bad luck with good command. But two negatives have made an even bigger negative for the 34-year-old who has become the poster boy for the White Sox's April bullpen plight.
First, let's look at Thornton's command. Comparing his fastball heat maps from 2010 and 2011, it's easy to see why he's struggling:
Heat maps via Fangraphs
Obviously, there's about 10 times more data for 2010 than 2011. But in 2010—and in his previous successful seasons—Thornton was able to locate his fastball inside on right-handers. In 2011, he's missed up and out over the plate a lot more (as shown by the lighter shade of blue). He hasn't been incapable of getting inside on righties, but he hasn't done it with the kind of consistency we're used to.
But Thornton's loss in confidence with his slider stems from his worse fastball command. As soon as the slider had to become more than a show-me pitch—thanks to his lessened effectiveness with his fastball—Thornton lost confidence in it. And that led him to go deep into his repertoire well and break out the changeup, likely just to try to give hitters another look.
What'll it take for Thornton to cease throwing his changeup? Better fastball command. With better fastball command, his slider can go back to its role as a show-me pitch and he won't feel the need to throw his least effective tertiary pitch.
But even if Thornton's fastball command doesn't return to its 2008-2010 level of pinpoint accuracy, he still could use some better luck. Thornton's .417 BABIP is outrageous and is bound to come down at some point. He's stranded just 36.8 percent of baserunners, a similarly outrageous percentage that won't stay that low. Opponents are making contact on about 15 percent more of Thornton's pitches this year than in 2010, and with Thornton's stuff eventually he's going to miss some bats.
And remember: he's pitched 4.2 innings this year. Given the precedent Thornton has set in the last three seasons, these 4.2 innings are most likely an anomaly. Let's wait until we have some larger sample sizes to jump off the Thornton bandwagon. Four and two-thirds half-bad, half-unlucky innings are hardly enough to undo three years of excellence.